Question

Select the living fossil from the following: (a) Culex (b) Silkworm (c) Lac insect (d) Limulus

Short answer

The living fossil among the given options is (d) Limulus.

Step-by-step solution

Identify the term

Firstly, understand what a living fossil is. It's an existing species of organism that closely resembles species otherwise known only from fossils and has few or no close living relatives. These species have all survived major extinction events because of their 'primitive' body plans, which have remained stable for over hundreds of millions of years.

Evaluate the options

Now consider each option individually: (a) Culex (a genus of mosquitoes), (b) Silkworm (a species of silk-producing worm), (c) Lac insect (a species of scale insect), and (d) Limulus (horseshoe crabs, a genus of marine and brackish water arthropods).

Apply knowledge and conclude

Knowing the definition of living fossil, apply that knowledge to the options. Among these options, the Limulus or horseshoe crabs are considered 'living fossils' because they have remained almost unchanged for over hundreds of millions of years, and have few or no close living relatives.

Key concepts

Extinction Events

Extinction events, often referred to as mass extinctions, are periods in Earth's history when a significant and widespread loss of species occurs in a relatively short geological time frame. These catastrophic events have shaped the evolutionary path of life on our planet. For example, the most famous extinction event is the Cretaceous-Paleogene (K-Pg) event approximately 66 million years ago, which led to the extinction of the dinosaurs, paving the way for mammals to rise in prominence.

Understanding extinction events is crucial because they highlight the resilience of certain organisms. Living fossils, such as the horseshoe crab, have intriguingly managed to survive such devastating periods while maintaining their primitive body plans. Thus, living fossils offer us a unique window into the past and help scientists understand how some species adapt and endure through extreme environmental changes.

Primitive Body Plans

Primitive body plans refer to the basic structural designs of an organism that have remained relatively unchanged through evolutionary history. These body plans are termed 'primitive' because they are similar to the form from which later organisms evolved. They provide essential clues about the early development of life forms on Earth.

Organisms with primitive body plans usually have simple structures and lack specialized adaptations that evolved in other species over time. Despite the term 'primitive', such body plans may actually reflect highly successful adaptations to stable environmental niches. Horseshoe crabs, often used as an example, possess a body plan that has allowed them to survive for hundreds of millions of years with few modifications, demonstrating that a 'simple' design can be incredibly efficient and durable in the face of changing climates and competition.

Horseshoe Crabs

Horseshoe crabs are a fascinating group of marine arthropods that have become emblematic of living fossils due to their ancient lineage and unchanging appearance. Today, four species of horseshoe crabs still roam the seabed as they have done since before the age of dinosaurs. With their hard carapaces and long, spiked tails, these creatures have become an iconic symbol of survival.

Horseshoe crabs have survived all five of the major extinction events that have occurred over the past 450 million years. This remarkable endurance is largely attributed to their versatile and effective body plan. Furthermore, they are of significant interest and value to medical science since their blue blood contains amebocytes, which are used to detect bacterial toxins in medical applications. As 'time travelers' from another era, horseshoe crabs remind us of the incredible continuity of life on Earth and how ancient designs still function effectively in our present-day ecosystems.